This invention relates to an article of flexible graphite sheet, having a coating of glassy carbon, which is fluid permeable in a transverse direction with enhanced isotropy with respect to thermal and electrical conductivity and enhanced resistance to chemical attack. This article can be used as an electrically conductive element in an electrical capacitor of the flow-through type. In a particular embodiment, natural cellulosic fibers are included in the glassy carbon coating and are carbonized and activated.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties which are highly directional e.g. thermal and electrical conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The natural graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Natural graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is at least 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is at least 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles which generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
This very considerable difference in properties, i.e. anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for improved gasket performance, if the resistance to fluid flow parallel to the major surfaces of the graphite sheet (xe2x80x9caxe2x80x9d direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet (xe2x80x9ccxe2x80x9d direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces (xe2x80x9ccxe2x80x9d direction) of the flexible graphite sheet, and substantially less in the direction parallel to and between the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction). In applications such as certain components for electrochemical cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (xe2x80x9ccxe2x80x9d direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction).
With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the upper and lower surfaces of the flexible graphite sheet is relatively high, while it is relatively very low in the xe2x80x9ccxe2x80x9d direction transverse to the upper and lower surfaces.
Another carbon based material having unique properties is glassy carbon.
As used herein and as described in U.S. Pat. No 5,476,679, the disclosure of which is incorporated herein by reference, glassy carbon is a monolithic non-graphitizable carbon with a high isotropy of the structure and physical properties and with a low permeability for gases and liquids. Glassy carbon typically also has a pseudo-glassy appearance. Glassy carbon can be formed from a non-graphitizing carbon-containing thermosetting resin such as synthetic or natural resins. Thermosetting resins that become rigid on heating and do not significantly soften upon reheating and are particularly effective. The principal groups of resins suitable for use in this invention are phenolics, polymers of furfural and furfuryl alcohol, as well as urethanes, which are minimally useful due to low carbon yields. The preferred phenolics are phenol-formaldehyde and resorcinol-formaldehyde. Furan based polymers derived from furfural or furfuryl alcohol are also suitable for use in this invention. The resin system should preferably give a carbon yield in excess of about 20% and have a viscosity below about 200-300 cps. In addition to solutions of phenolics in furfural and furfuryl alcohol, straight furfural or furfuryl alcohol can be used with a catalyst. For example, a solution of furfural and an acid catalyst could be coated on a surface and then cured and carbonized to form glassy carbon.
Glassy carbon can prevent diffusion of contaminants and since glassy carbon is harder than graphite, glassy carbon will also provide protection from flaking, scratching and other defects and glassy carbon, unlikely glass itself, is a relatively good conductor.
The aforedescribed materials, in combination, are advantageously employed in a flow-through capacitor described in U.S. Pat. No. 5,779,891, the disclosure of which is incorporated herein by reference.
The flow-through capacitor, used in the separation and other treatment of fluids, and more fully described hereinafter, comprises at least one anode and at least one cathode adapted to be connected to a power supply, the capacitor arranged and constructed for use in the separation, electrical purification, concentration, recovery or electrochemical treatment or breakdown of solutes or fluids.
The capacitor includes one or more spaced apart pairs of anode and cathode electrodes incorporating a high surface area electrically conductive material and characterized by an open, short solute or fluid flow path, which flow paths are in direct communication with the outside of the capacitor.
In accordance with the present invention, a graphite article is provided comprising a compressed mass of expanded graphite particles in the form of a sheet having parallel, opposed first and second surfaces, at least one of the parallel opposed surfaces having an adherent coating of glassy carbon. The coated sheet, in at least a portion thereof, has a plurality of transverse fluid channels passing through said sheet between the parallel, opposed first and second surfaces, the channels being formed by mechanically impacting a surface of the sheet to displace graphite within the sheet at a plurality of predetermined locations to provide the channels with openings at the first and second parallel opposed surfaces. In a preferred embodiment, the inner surface of the channels have an adherent coating of glassy carbon whereby chemical and erosive attack at the channel sidewalls is avoided. The article of the present invention is useful as an electrically conductive backing material and electrode for use in xe2x80x9cflow throughxe2x80x9d type capacitors.